The present invention relates to an optical head device for reproducing signals recorded on an optical recording medium and, more particularly, it relates to an optical head device capable of controlling focusing and tracking processes with high accuracy.
Various kinds of digital audio machines have been developed these days. Since excellently reproduced signals can be obtained independently of the properties of the recording medium, particularly with the device in which an audio signal is digitally recorded and reproduced using the PCM technology, it has been practically used as the disk recording device of the optical type, for example. With this disk record device of the optical type, for example, an optical disk having a diameter of 12 cm and a thickness of 1.2 mm and coated by a thin metal film is used, and pits which correspond to the digital information of audio signal are recorded on the thin metal film. These pits are recorded at a certain linear veloctity, driving the disk at a variable rotation speed of 200 to 500 rpm. In a case where the pitch of the disk is 1.6 .mu.m, stereo signals can be recorded on a face of the disk for about one hour.
When recorded signals are reproduced from the disk with this optical type head device, it is necessary that the tracking of the optical head and the focusing of the reproducing optical system are controlled with high accuracy. Further, it is necessary that parts of the head have a simple construction and are associated with one another to achieve a reliable operation. Furthermore, it is necessary from practical viewpoint that the head device is small in size and the current consumption of the device is low.
The conventional optical head device is shown in FIG. 1. A fixing member 10 is fixed to a body of the optical head device. One end of each of the four horizontal plate springs 14 is attached to the fixing member 10. A movable member 12 is fixed to the other ends of the plate springs 14. One end of each of the four plate springs 18 with its face remaining vertical is fixed to the movable member 12. A holder 16 to which an objective lens 20 is attached is fixed to the other ends of the plate springs 18. The holder 16 and the objective lens 20 are therefore supported by the plate springs 14 and 18 through the movable member 12. The holder 16 and the lens 20 can move two-dimensionally in the focusing and tracking direction 2 and 4 thanks to the elastic deformation of the plate springs 14 and 18.
With the optical head device of this type, it is necessary that focusing and tracking control be achieved with stability even when any impact is applied from outside the device. It is necessary for this reason that the servo band be higher than 2 KHz. It is therefore necessary that the operational characteristic (or suspension characteristic), which is determined by the elasticity of the plate springs and the weight of the movable member, holder and the like, has a secondary natural frequency, if any, higher than 8KHz in the tracking and focusing directions. In addition, the DC gain must be high to stabilize the servo system. It is necessary for this reason that the suspension characteristic has a primary natural frequency which ranges from 0 to 40 Hz.
In the case of the optical head device embodied according to the conventional technology, however, it is extremely difficult to make the secondary natural frequency higher than 8 KHz when the primary natural frequency is about 40 Hz because the weight of the movable member 12 is large. The servo band is narrow accordingly. When the holder 16 is moved in the focusing and tracking directions 2 and 4, the plate springs 14 and 18 deform elastically to shift the holder 16 in the signal reading direction (perpendicular to the directions 2 and 4), thereby causing the jitter (jiggle). In order to reduce this jitter, it is necessary that the plate springs 14 and 18 be made long. This makes the device large in size.
Another conventional optical disk device is shown in FIGS. 2 and 3. A cylindrical fixing member 22 is fixed to a body of the device. Outer ends of paired focusing diaphragm springs 28 separated vertically from each other are fixed to the inner circumferential face of this fixing member 22. A cylindrical movable member 24 is fixed to the inner ends of the springs 28. Trunk members 32 are fixed to the lower end of the movable member 24. A pair of tracking plate springs 30 parallel to each other are erected on the trunk members 32. A holder 26 for supporting a lens 20 is held between the upper ends of the plate springs 30. The holder 26 can move in the focusing direction 2 thanks to the elastic deformation of the diaphragm springs 28 and in the tracking direction 4 thanks to the elastic deformation of the plate springs 30.
When the tracking plate springs 30 are shifted in the tracking direction 4, their stiffness in the focusing direction 2 decreases remarkably in the case of the device having the above-described arrangement. Therefore, the secondary natural frequency in the focusing process by means of the plate springs 30 decreases remarkably, and the secondary natural frequency becomes lower than 8 KHz, similar to the case shown in FIG. 1. The servo band is narrow accordingly and it is difficult to stably operate the device.
A further optical head device is shown in FIGS. 4 and 5. A lens 20 is supported by a holder 44, which is also supported at the both sides thereof by means of a pair of support mechanisms. The support mechanism has a fixing member 34 fixed to the device body, and a movable member 38 for fixedly supporting the holder 44. The fixing member 34 and movable member 38 are vertically separated from each other and connected to a trunk member 36 by means of plate springs 40 whose faces are kept parallel to each other. FIGS. 6 through 8 are side views only showing the support mechanism in order to explain the operation of the device shown in FIGS. 4 and 5. When the plate springs 40 bend as shown in FIG. 7 from their neutral state (in FIG. 6), the movable member 38 and the holder 44 can be moved in the focusing direction 2. On the other hand, when the plate springs 40 bend as shown in FIG. 8, the movable member 38 and the holder 44 can be moved in the tracking direction 4.
In the case of this device, the movable member 38 can be held and moved in the focusing and tracking directions 2 and 4. It is easy for this device to obtain a wide servo band. In order to use the plate springs 40 within the limit of their elasticity, however, it is necessary that they be made long. This makes the device large in size. In addition, when the trunk member 36 is shifted from its neutral state (FIG. 6), the stiffness of the support mechanism decreases in a direction in which the movable member 38 falls, thereby causing resonance of this falling mode. This secondary resonance makes the servo system unstable for controlling the movement of the movable member 38.